The development of photochromic or phototropic glasses, as such have been variously termed, was founded in U.S. Pat. No. 3,208,860. As is explained in that specification, a photochromic glass becomes darker (changes color) when exposed to actinic radiation, most commonly ultraviolet radiation, and fades or returns to its original color when the actinic radiation is removed. That patent teaches the utility of silver halide crystals, viz., silver chloride, silver bromide, and silver iodide, and postulates an explanation of the mechanism underlying the photochromic behavior displayed by those glasses containing silver halide crystals. The patent is drawn generally to silicate-based glasses, with the preferred compositions being encompassed within the alkali metal oxide-Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 system. Thus, the preferred base compositions consist essentially, expressed in weight percent on the oxide basis, of about 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3, and 40-76% SiO.sub.2, wherein R.sub.2 O is selected from the group of 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O, the sum of the base glass ingredients constituting at least 85% of the total composition. The patent further observes the advantage of including small quantities of low temperature reducing agents, such as SnO, FeO, CuO, As.sub.2 O.sub.3, and Sb.sub.2 O.sub.3, to improve the photochromic characteristics of the glass. In order to impart photochromic behavior to the glass, at least one halide must be present in the glass in at least the effective amount of 0.2% Cl, 0.1% Br, and 0.08% I, and silver must be present in the minimum of 0.2% where Cl is the effective halide, 0.05% where Br is the effective halide, and 0.03% where I is the effective halide. Finally, where a transparent photochromic glass is desired, the glass must not contain more than 0.7% silver or more than 0.6% total of the three halides.
The most extensive use to date of photochromic glass has been in the field of ophthalmic lenses, both as prescription lenses and as non-prescription sunglasses. Prescription lenses, marketed under the trademark PHOTOGRAY.RTM., have constituted the greatest segment of the commercial sales. That glass is encompassed within the disclosure of U.S. Pat. No. 3,208,860, supra, and has the approximate analysis recited below in weight percent:
______________________________________ SiO.sub.2 55.6% B.sub.2 O.sub.3 16.4 Al.sub.2 O.sub.3 8.9 Li.sub.2 O 2.65 Na.sub.2 O 1.85 K.sub.2 O 0.01 BaO 6.7 CaO 0.2 PbO 5.0 ZrO.sub.2 2.2 Ag 0.16 CuO 0.035 Cl 0.24 Br 0.145 F 0.19 ______________________________________
Inasmuch as PHOTOGRAY.RTM. glass is the result of compromises drawn between photochromic properties, ophthalmic properties, the capability for being chemically strengthened, and melting and forming behavior, considerable research has been undertaken to produce a glass having improved photochromic properties while still retaining the other necessary characteristics to be a practical commercial glass.
One circumstance which must be remembered in any such research is the fact that the dynamics of photochromic behavior exhibited by glasses are dependent to a greater or lesser extent upon the temperature of the glass and the intensity of the actinic radiation incident thereon. Hence, as a general rule, where other parameters are maintained constant, a photochromic glass will darken to a lower transmission when subjected to actinic radiation at lower temperatures and will fade more slowly when the actinic radiation is removed. Furthermore, the intensity of solar radiation can vary widely depending upon the season of the year, the location of the exposure (angle of declination of the sun), cloud cover, snow cover, air mass value, etc.
Some photochromic glass compositions have been produced which, in 2 mm thickness, will darken to a transmittance of less than 1% when exposed to solar radiation at low temperatures, e.g., -18.degree. C. (0.degree. F.). Such glasses do not conform to the common commercially-marketed, fixed tint sunglasses which customarily exhibit a transmittance of about 15%. Moreover, darkening to such low values may pose a substantial hazard to the wearer of ophthalmic lenses made from such glasses.
This relation of photochromic behavior to temperature has been termed the temperature dependence of a glass and refers to the loss of darkening exhibited by a glass as the temperature thereof is raised. This loss of darkening is due to the increase in thermal fade rate as the temperature of the glass is raised and can be very significant even over the limited temperature variations observed under ambient conditions, viz., a range from -18.degree. C. to 40.degree. C. (0.degree.-100.degree. F.).
It must also be borne in mind that the transmittance of a darkened photochromic glass sample is related in part to the thickness thereof. Thus, because of the absorption of the actinic radiation by the photochromic particles in the glass, the known photochromic glasses do not strictly adhere to Bouguer's Law. This circumstance assumes practical significance since, whereas the majority of ophthalmic lenses produced has a thickness dimension of 2-3 mm, there are some ophthalmologic conditions which demand lenses of greater thickness. Obviously, if Bouguer's Law held, such thick lenses (4 mm and greater) would manifest very low transmittances in the darkened state, especially at low temperatures. Nevertheless, even though Bouguer's Law is not strictly applicable, thick glasses do get darker than thin glasses.
Accordingly, because of those factors, the present applicants have deemed it advisable to restrict the minimum darkened transmittance of their glasses in 2 mm thickness to about 15% at low temperatures.
From the considerable experience gained through the years with photochromic glasses in the ophthalmic field, several criteria have been formulated therefor which would be highly desirable to achieve, these criteria being in addition to the necessary melting and forming capability as well as the conventional physical properties demanded in non-photochromic ophthalmic ware.
First, a glass which in 2 mm thickness at room temperatures (20.degree.-30.degree. C.) will demonstrate a luminous transmittance of at least about 90% before exposure to actinic radiation but which, when irradiated with actinic radiation, e.g., bright outdoor sunlight, will darken to a transmittance of less than 40%.
Second, a glass which in 2 mm thickness at room temperatures will fade very rapidly when removed from the incident actinic radiation; i.e., the glass will fade to a transmittance of at least 80% in less than two hours.
Third, a glass which in 2 mm thickness at -18.degree. C. will darken to a transmittance of not less than about 15%.
Fourth, a glass which is capable of being strengthened via either thermal tempering or chemical strengthening while maintaining the desired photochromic properties.
Fifth, a glass having a base composition capable of refractive index adjustment while retaining the desired photochromic properties.
For the purposes of the present description, the luminous transmittance of a glass is defined as the value Y delineated in terms of the 1931 C.I.E. trichromatic colorimetric system utilizing the light source Illuminant C. This colorimetric system and light source are described by A. C. Hardy in the Handbook of Colorimetry, Technology Press, M.I.T., Cambridge, Massachusetts (1936).
This research to produce glasses displaying improved photochromic properties has led to investigations of other base glass systems. For example, U.S. Pat. No. 3,834,912 discloses glasses having base compositions within the PbO-B.sub.2 O.sub.3 field, i.e., the glasses consist essentially, by weight, of 14.2-48% B.sub.2 O.sub.3, 29-73% PbO, 0-15% alkaline earth metal oxides, 0-8% alkali metal oxides, and 0-23% ZrO.sub.2, Al.sub.2 O.sub.3, and/or ZnO. AgCl, AgBr, and/or AgI crystals impart the desired photochromic properties and up to 0.8% CuO and/or up to 0.05% Cr.sub.2 O.sub.3 are noted as having utility as sensitizers. U.S. Pat. No. 3,876,436 is directed to base glass compositions in the Al.sub.2 O.sub.3 -P.sub.2 O.sub.5 field, i.e., the glasses consist essentially, by weight, of at least 17% P.sub.2 O.sub.5, 9-34% of Al.sub.2 O.sub.3, not more than 40% SiO.sub.2, not more than 19% B.sub.2 O.sub.3, and at least 10% alkali metal oxides. Again, AgCl, AgBr, and AgI crystals provide the photochromic properties.
However, the lead borate-based glasses frequently encounter melting and forming problems and can be deficient with respect to chemical durability. The phosphate-based glasses can also be subject to less than satisfactory durability and, whereas some compositions exhibit fast fading characteristics, those fast fading glasses are observed to darken below 5% transmittance in 2 mm thickness when subjected to actinic radiation at -18.degree. C.
Consequently, because of the practical advantages of glasses having compositions within the aluminoborosilicate base field with regard to physical properties other than photochromic behavior, as well as the relative ease in melting and forming, much activity has centered around attempts to improve the photochromic behavior in glasses having compositions within that base. Such research has fostered the trial of photochromic ingredients other than the silver halides. For example, U.S. Pat. No. 3,325,299 is drawn to silicate glasses and, preferably, aluminoborosilicate glasses wherein copper and/or cadmium halide crystals constitute the photochromic ingredients. Nevertheless, the effectiveness of materials other than the silver halides to provide the desired photochromic characteristics has not as yet supported a commercial product. Accordingly, the bulk of the investigative effort has been directed to silver halide-containing glasses having base compositions within the alkali metal aluminoborosilicate system. A recent illustration of such activity is shown in French Pat. No. 2,320,913.
That patent discloses photochromic glasses which are asserted to be particularly suitable for ophthalmic lenses to be worn by automobile drivers at twilight or at night. The glasses described therein are stated to have a saturation transmittance to visible light in 2 mm thickness of between 40-45% and are claimed to be capable of fading from the darkened state at 20.degree. C. to a visible transmittance of at least 80% after no more than 30 minutes. The glasses have base compositions within the ranges of, as expressed in weight percent:
______________________________________ SiO.sub.2 45-62% B.sub.2 O.sub.3 9-22 Al.sub.2 O.sub.3 4-14 ZrO.sub.2 0-4.2 MgO 0-2.8 BaO 3-10.5 Li.sub.2 O 0.8-4.6 Na.sub.2 O 0.3-10 K.sub.2 O 0-10 F 0-1 ______________________________________
The most critical feature of the patented compositions is declared to reside in maintaining the proportions of Ag.sub.2 O, CuO, PbO, Cl, and Br within the analyzed ranges recited below in weight percent:
______________________________________ Ag.sub.2 O 0.195-0.265 CuO 0.026-0.038 PbO 2.76-5.50 Cl 0.220-0.450 Br 0.080-0.200 ______________________________________
Two factors relating to halide contents are also noted in that patent. First, that the fade rate of the claimed glass compositions is not enhanced through increased proportions of bromide and/or chloride. Second, an increase in bromide and/or chloride content adversely affects the darkening tendency of the glass, i.e., the glasses do not darken to as low an optical transmittance.
Another recent disclosure concerned with silver halide-containing glasses having base compositions within the alkali metal aluminoborosilicate system is U.S. Pat. No. 4,018,965. That patent is expressly drawn to glass compositions especially suitable for chemical strengthening and demonstrating the necessary melting and forming capabilities for sheet drawing. The glasses recited therein have the base compositions recited below in weight percent on the oxide basis:
______________________________________ SiO.sub.2 54-66% Al.sub.2 O.sub.3 7-15 B.sub.2 O.sub.3 10-25 Li.sub.2 O 0.5-4 Na.sub.2 O 3.5-15 K.sub.2 O 0-10 PbO 0-3 Ag 0.1-1 Cl 0.1-1 Br 0-3 F 0-2.5 CuO 0.008-0.16 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 6-16 ______________________________________
The patent also describes the optional inclusion of up to 1% total of transition metal oxide colorants and up to 5% total of rare earth metal oxide colorants.
Such glasses do indeed exhibit excellent modulus of rupture values after chemical strengthening and the compositions can be so adjusted as to provide exceptional sheet forming capabilities. However, an improvement in photochromic behavior with respect to fade rate would be desirable. Moreover, the patent makes no reference whatever to the temperature dependence displayed by the glasses so, obviously, does not define glass compositions which demonstrate relative independence of temperature effects.
Another borosilicate photochromic glass designed for prescription ophthalmic lenses has been marketed under the name PHOTOVITAR. The glass has the approximate analysis reported below in weight percent:
______________________________________ SiO.sub.2 54.0 B.sub.2 O.sub.3 16.5 Al.sub.2 O.sub.3 8.9 Li.sub.2 O 2.37 K.sub.2 O 1.88 MgO 2.42 BaO 9.7 PbO 0.6 ZrO.sub.2 1.9 Ag 0.14 F 0.19 Cl 0.59 Br 0.18 CuO 0.015 ______________________________________
That glass demonstrates good darkening and fading characteristics in the ranges of room temperature but fails to darken a desired amount at higher temperatures and exhibits a transmittance of less than 15% at -18.degree. C.
Yet another disclosure describing glass compositions assertedly demonstrating very fast fading capabilities is found in U.S. Pat. No. 4,102,693. The glasses are stated to exhibit a half fading time of not more than 60 seconds, half fading time being defined as the period required to fade from the darkened state to a condition in which half of the lost light transmittance has been restored. The compositions are free from barium and consist essentially, in weight percent, of
______________________________________ SiO.sub.2 31-59% B.sub.2 O.sub.3 18-28 Al.sub.2 O.sub.3 8-20 Li.sub.2 O 0-3 Na.sub.2 O 0-8 K.sub.2 O 0-16 Li.sub.2 O+Na.sub.2 O+K.sub.2 O 6-16 Ag.sub.2 O 0.05-4 Cl 0.04-0.5 Br 0-1.0 F 0-0.2 Cl+Br+F 0.13-1 CuO 0-1 ______________________________________
A number of optional components is mentioned to modify such physical properties of the glass as refractive index, although none of the working examples reported has a refractive index as high as 1.523, the level required for ophthalmic applications.
As is evident from the above-recited compositions, the range of glass suitable for the purposes of the patent is very broad. This is quite understandable since the disclosure is explicitly directed to glasses demonstrating extremely fast fading capabilities with no regard to other facets of photochromic behavior, e.g., the phenomenon of temperature dependence. Hence, the patent does not indicate the temperature at which the studies of photochromic behavior exhibited by the exemplary glasses were conducted. No comparative data measured at different temperatures are provided. Accordingly, no information can be gleaned therefrom as to means for producing photochromic glasses displaying low temperature dependence.
Still another description of index corrected photochromic glass compositions which are characterized by extremely fast fading capabilities is found in U.S. Pat. No. 3,957,499. The glasses are asserted to fade so rapidly that at least 65% and up to in excess of 80% of the optical density gained during darkening is lost within a five-minute fading interval. The glasses consist essentially, expressed in weight percent on the oxide basis as calculated from the batch, of:
______________________________________ SiO.sub.2 49-60 Al.sub.2 O.sub.3 2-9 B.sub.2 O.sub.3 15-18 Na.sub.2 O 6-12 ZrO.sub.2 9-18 Ag 0.5-0.9 Cl 0.5-0.8 CuO 0.01-0.03 PbO 0.3-1 ______________________________________
Optional ingredients include 0-6% K.sub.2 O, 0-3% Li.sub.2 O, 0-4% BaO, 0-1% MgO, 0-2% TiO.sub.2, 0-0.5% Br, and 0-0.5% I. The crux of the invention is observed to be the use of increased quantities of ZrO.sub.2 to replace at least part, if not all, of the BaO and PbO conventionally utilized in controlling the refractive index of the glass for ophthalmic purposes.
Nevertheless, again there is no discussion regarding the temperature dependence of the photochromic properties demonstrated by the glasses. No comparative data over a range of temperatures are provided.
In summary, none of the disclosures has provided any substantive teaching regarding temperature dependence which would help to satisfy the several criteria outlined previously, so research has been constant to produce glasses demonstrating even better physical, optical, and photochromic properties. This research has led to the development of sophisticated apparatus and tools to assist in the screening and understanding of photochromic glasses.
Thus, inasmuch as it was known that photochromic glasses were sensitive to radiations in the ultraviolet and low visible portions of the spectrum, an ultraviolet lamp has long been customarily employed as a convenient source of actinic radiation to test the photochromic behavior of glass specimens. Nevertheless, it has been appreciated that frequently there was poor correlation between the data secured with the ultraviolet lamp and the results observed through solar exposure outdoors. Accordingly, in order to achieve correlations with outdoor solar exposure, a "solar simulator" was devised.
The solar simulator apparatus, described in U.S. Pat. No. 4,125,775, is grounded in a 150 watt xenon arc source fitted with a filter to modify the spectral output thereof so as to closely approximate the solar spectrum, especially in the ultraviolet, blue, and red portions. The infrared region of the spectrum is attenuated with a layer of water of sufficient thickness to provide equal irradiance to that of the sun, but without great concern for its spectral distribution in that region.
The intensity of the arc source was adjusted such that the amount of darkening was identical to that of a number of commercially-available photochromic glasses, including PHOTOGRAY.RTM. lens blanks, darkened outdoors at noon during a cloudless early summer day in Corning, New York (air mass value of about 1.06). Numerous experimental photochromic glasses of widely-variant compositions were also subjected to the solar simulator and to outdoor sunlight. Excellent overall agreement was observed in comparisons between the data obtained.
In order to continuously monitor the darkened transmittance of the specimens, each sample was interrogated with a chopped beam of light from a tungsten-halogen lamp detected by a PIN silicon photodiode whose output was demodulated by a lock-in amplifier. A composite color filter was placed into the beam to approximate the luminous response of the human eye under Illuminant C, as defined by C.I.E.
For measurements conducted at 26.degree.-27.degree. C. and 37.degree.-38.degree. C., the apparatus was interfaced to a PDP-11/04 computer (marketed by Digital Equipment Corporation, Maynard, Massachusetts) to enable automatic sample change, temperature selection, event sequencing, and data collection, storage, reduction, and retrieval with a minimum of operator's involvement.
Measurements at -18.degree. C., 0.degree. C., 20.degree. C. and 40.degree. C. were manually conducted with the samples mounted in a vacuum chamber with fused silica windows allowing entrance of the darkening light and passage of the interrogation beam. This permitted attaining temperatures substantially deviant from ambient and prevented condensation of atmospheric moisture on the sample at the lower temperatures. The sample holder consisted essentially of a copper plate, with a hole in the center for passage of the interrogation beam, which was heated or cooled by flowing a gaseous stream of the desired temperature through an attached heat exchange channel. The temperature of the gas was controlled by passing it through a coil immersed into liquid nitrogen and then over an electrically heated element controlled by a thermocouple impinging on the sample surface. The sample was mounted on the holder by means of a thermally conductive paste. The transmittance of the sample during darkening and fading cycles was recorded on a strip chart recorder.
Exposure of PHOTOGRAY.RTM. lens blanks and PHOTOVITAR glass samples of 2 mm thickness to the solar simulator yielded the following average values recited below. T.sub.D designates the darkened transmittance and T.sub.F5 indicates the transmittance of the sample five minutes after removal of the sample from exposure.
__________________________________________________________________________ PHOTOVITAR PHOTOGRAY.RTM. Exposure Exposure Exposure Exposure Temperature Time T.sub.D T.sub.F5 Temperature Time T.sub.D T.sub.F5 __________________________________________________________________________ 40.degree. C. 20 min. 62% 86% 40.degree. C. 20 min. 58.5% 76.5% 20.degree. C. 30 min. 41% 70% 20.degree. C. 20 min. 47% 61% 0.degree. C. 30 min. 23.5% 46.5% 0.degree. C. 20 min. 37.5% 47.5% -18.degree. C. 60 min. 13% 22% -18.degree. C. 20 min. 31.5% 37.5% __________________________________________________________________________
Several general conclusions can be drawn from the above data. Thus, the glasses darken to a lower transmittance when exposed at lower temperatures. The PHOTOVITAR glass does not darken to a great extent at high temperatures, but darkens to very low values at low temperatures. The PHOTOVITAR glass exhibits more rapid fading than PHOTOGRAY.RTM. lens blanks, but neither glass fades very rapidly at low temperatures. This sluggishness in fade rate at low temperatures, however, may not be of significant practical importance since, in many instances, the glass will be warming up while it is fading. For example, the wearer will be coming indoors from being outdoors on a cold day and, as can be seen from the above comparisons, the fade rate increases as the temperature rises. Finally, the PHOTOGRAY.RTM. lenses display less temperature dependence than the PHOTOVITAR glass.